Overview
HDLC is a bit-oriented synchronous data-link layer protocol developed by the International Organization for Standardization (ISO). It specifies a data encapsulation method on synchronous serial links using frame characters and checksums.
By default, synchronous serial lines use the HDLC serial encapsulation method, which provides the synchronous framing- and error-detection functions of HDLC without windowing or retransmission.
Cisco HDLC monitors line status on a serial interface by exchanging keepalive request messages with peer network devices. It also allows routers to discover IP addresses of neighbors by exchanging Serial Link Address Resolution Protocol (SLARP) address-request and address-response messages with peer network devices.
The system responds to a SLARP address-request message from a remote peer with a SLARP address-response message, which indicates that it cannot participate in a SLARP session.
Cisco HDLC is compatible with Cisco Systems Cisco-HDLC protocol, the default protocol for all Cisco serial interfaces.
Line Modules
For a list of the specific line modules and corresponding I/O modules that support Cisco HDLC, see ERX Installation and User Guide, Chapter 11, Protocol Support.
Framing
The system supports the following framing features:
Error Frames
All Cisco HDLC error frames are discarded.
SLARP Keepalive
One feature of Cisco HDLC is the exchange of keepalive messages. A keepalive message is a signal from one endpoint to the other that the first endpoint is still active. Keepalives are used to identify inactive or failed connections.